You can find here damn simple and juicy tool for video rotating

Transforming your movie, please be patient (remaining time: )

Preparing your movie for further processing

Vertical video - what does it mean and why did you do this again?

It means that you grabbed your phone badly while filming.
It is obvious now, but when we are excited and trying to catch some moment quickly, we are holding the phone in the most natural way.
This way can be called portrait mode.
Finally you will have video which needs to be fixed and you are in the right place.